The present invention relates to a super-pressure balloon for flying at a high altitude such as an observation balloon in the stratosphere and the like. Particularly, the present invention provides a super-pressure balloon equipped with a gas balloon having a structure capable of bearing a higher internal pressure and capable of flying at a higher altitude for a longer time, and a method of manufacturing this super-pressure balloon.
There has conventionally been a large scientific observation balloon for flying in the stratosphere at an altitude of about 30 km to 40 km, for example. Such a high-altitude balloon has a gas bag inflated as the balloon goes up in the sky, and the capacity of the gas bag becomes a maximum when the balloon reaches a maximum altitude. At this time, a gas of free buoyancy of about 10% of the total weight is discharged from a discharge opening formed at a lower part of the gas bag, so that the balloon flies by maintaining a constant height.
However, as the temperature of the gas within the gas bag lowers at sunset, the buoyancy of the gas decreases by about 7 to 10% of the total buoyancy, and the height of the balloon is lowered. Accordingly, in order to maintain the height of the balloon at a constant level after sunset, it is necessary to compensate for a reduction in the buoyancy by dropping the ballast. Therefore, in the case of a balloon for flying over long hours such as several days, for example, it is necessary to load in advance a large volume of ballast on the balloon for maintaining the high altitude as described above.
For example, in the case of a scientific observation balloon for making a round trip on the schedule of about ten days after a start up of the flying at the South Pole by placing the balloon in the circumferential wind blowing in the stratosphere in summer season, approximately a half of loaded weight of 300 kg was for the ballast weight for maintaining the high altitude.
With the above background, it has been desired to decrease the above-described ballast consumption to enable the balloon to fly over long hours and to increase the payload. As means for satisfying this desire, there is available what is called a super-pressure balloon equipped with a pressure-resistant gas bag.
This super-pressure balloon is equipped with a pressure-resistant gas bag that can bear an internal pressure, so that even after the capacity of the gas bag has become a maximum after reaching a maximum height, the balloon does not discharge gas for the ascending buoyancy but maintains a horizontal flying at a constant height after once the buoyancy decreases due to a reduction in the atmospheric density, while maintaining a maximum capacity of the gas bag and an internal gas pressure. According to this super-pressure balloon, the maximum capacity of the gas bag does not change except the gas pressure within the gas bag is lowered when the temperature of the gas lowers after sunset, so that the balloon can maintain the horizontal flying while keeping the maximum height without dropping the ballast. Accordingly, it becomes possible to make the balloon fly over a longer period of time without consuming a large volume of ballast unlike the conventional balloon, and the payload also increases.
In order to achieve the super-pressure balloon as described above, it is necessary to provide a pressure-resistant gas bag that can endure the internal pressure. For increasing the pressure resistance of the gas bag, a film material of lighter weight and higher strength may be used. However, with a film material currently developed, it has been difficult to manufacture a large pressure-resistant balloon having a gas bag of 100,000 cubic meters for capacity and about 100 m for radius.
Therefore, in order to provide a super-pressure balloon of a larger size and a larger capacity, it is necessary to develop a structure of the gas bag that can bear a higher internal pressure as well as to develop a film material.
For facilitating the understanding of the present invention, a general structure of the balloon will be explained below with reference to FIGS. 16 to 18. In the drawings, a reference numeral 1 denotes a gas bag, and helium or the like is filled within this gas bag 1, to generate buoyancy. Payload including an observation device 2 and others is loaded on this gas bag 1. In the actual balloon, various kinds of control devices for carrying out gas discharging and ballast dropping, etc. are loaded, but they are omitted from the drawings.
The gas bag 1 has a schematically spherical shape, and is structured by connecting a large number of gores 3 that are the gas bag 1 vertically and equally divided into N spindle-shaped film pieces, as shown in FIG. 18. These gores 3 are formed by a light-weight and high-strength film material such as a woven cloth or a plastic film, with both side edge parts of the gores 3 mutually sewed or connected together, thereby to structure the gas bag 1. Further, load tapes 4 that can endure a high tension are sewed along the sewing or connection lines of these gores 3. These load tapes 4 increase the mutual connection strength of the gores 3, disperse the load of the observation device 2 and others and transmit this load to the gores 3, and also maintain the shape of the gas bag 1 in a predetermined shape.
Usually, a shape called Natural-Shaped Balloon is used for this gas bag 1. This Natural-Shaped Balloon is a shape which is so prescribed that tension is generated only in a vertical direction of a film material, that is, in the meridian direction, in a state that the buoyancy of the inside gas, the weight acting upon each part of the film material and others are balanced, and no tension is generated in a circumferential direction orthogonal with this vertical direction. In other words, the following relationship is obtained: EQU Tm=P.multidot.Rm+dWm (1)
where, Tm represents a tension in a vertical direction of an optional part of the film material, P represents a pressure acting upon the film material, Rm represents a radius of curvature of this part of the film material, and dWm represents a component force in a film tangential direction of the gravity acting upon a fine part of the film material.
In this case, the pressure P acting upon the film material at a position in a vertical direction y becomes a sum of a pressure generated by a difference d .rho. between a density of the gas within the gas bag and a density of the atmospheric air and a bias pressure P0 at a bottom part of the gas bag, with a gravity acceleration given as g. This relationship is given by the following expression: EQU P=P0+d.rho..multidot.g.multidot.y (2)
When a shape of the gas bag is expressed by a function y=f (x), the radius of curvature Rm in the vertical direction of the film material is given as follows: EQU Rm=(1+y'2)3/2/y" (3)
Accordingly, by preparing a differential equation of y by using the above (3) in the expression (1) and by integrating from the bottom part of the gas bag while building the dWm component into the tension Tm, it is possible to obtain the Natural-Shaped Balloon of the gas bag.
In the present specification, it is assumed that the Natural-Shaped Balloon is a shape that is determined by the above relationships (1) to (3) and, as described above, that is so prescribed that tension is generated only in a vertical direction of a film material, that is, in the meridian direction, in a state that the buoyancy of the inside gas, the weight acting upon each part of the film material and others are balanced, and no tension is generated in a circumferential direction orthogonal with this vertical direction.
When it is assumed that the film material that structures the gores 3 does not expand at all, the shape of the cross-sectional surface of the gas bag 1 becomes polygonal, with the cross-sectional shape of each gore 3 forming a linear shape represented by a two-dot chain line 3a, as shown in FIG. 17. However, as the film structuring these gores 3 expands in actual practice, the cross-sectional shape of the gas bag 1 becomes circular as represented by a solid line in FIG. 17 due to the inside gas pressure. Further, when these gores 3 are further stretched, each gore 3 swells out in approximately an arc shape as represented by a broken line 3b in FIG. 17, and load is applied to each load tape 4 to swell it out. However, this load is supported by the tension of the load tape 4.
When the internal pressure is applied to the gas bag 1, the tensile stress generated to the film material becomes proportional to the radius of curvature of the swell-out of this film material. Accordingly, when the swell-out 3b of each gore 3 shown in FIG. 17 is set larger, that is, when the radius of curvature of each gore 3 is set smaller, the pressure resistance of the gas bag against the internal pressure becomes higher, and this is advantageous in manufacturing the above-described super-pressure balloon.
However, according to the conventional balloons, there has been a limit to the expansion or stretching of the film material that structures the gores 3 although the swell-out of the gores 3 depend on the stretch of the film material. Therefore, it has not been possible to make larger the swell-out of the gores 3, or it has not been possible to make sufficiently smaller the radius of curvature.
As a method of making larger the swell-out of the gores 3, there is a one, as shown in FIG. 19, for cutting out portions 3c represented by shaded lines in the gore 3 and then sewing or connecting together the left edge parts of the gore 3 to form a three-dimensionally shaped gore 3. These gores 3 are sewed or connected together, in a manner as explained above, to form a gas bag.
A balloon obtained by this method as a result, however, has a large number of sewing lines and connection lines in each gore 3. Therefore, the strength of each gore is lowered and the weight of each gore 3 increases. Further, there is also an increasing risk of generation of leakage and others, which leads to a reduction in reliability. This balloon also has an extremely increased number of manufacturing processes and involves an inconvenience of increased manufacturing cost.